Reaction between an ethylene oxide (e.g. ethylene oxide and 1,2-propylene oxide) and an aromatic carboxylic acid is known to proceed at a temperature upward from 50.degree. C. and elevated pressure to maintain the ethylene oxide in the liquid phase to produce the corresponding 2-hydroxyethyl ester of the aromatic carboxylic acid without the attendant formation of by-product water as is the case when the corresponding glycol is used to esterify the acid. Reaction at 50.degree. C. is slow but as would be expected, the reaction rate increases as reaction temperature is increased above 50.degree. C. but reactions above 180.degree. C. require apparatus to withstand the increased pressure necessary to maintain liquid phase conditions. Also as reaction temperature is increased there is an increased tendency of diglycol ether ester (e.g. 2-hydroxyethoxyethyl ester from ethylene oxide) formation by reaction between the ethylene oxide and the 2-hydroxyethyl ester. The diglycol ether ester (hereafter DGEE) of the aromatic carboxylic acid contaminates the desired 2-hydroxyethyl ester and is difficult to remove therefrom. The formation of DGEE contaminant is especially undesirable in 2 -hydroxyethyl and 2-hydroxy-2-methylethyl esters of isophthalic acid, terephthalic acid and 1,1,3-trimethyl-3-(p-carboxyphenyl)-5-indane carboxylic acid which otherwise in the absence of DGEE can alone or in admixture be polycondensed to a fiber and film forming high molecular weight polyester. The presence of the DGEE causes formation of lower molecular weight polyester species which lower the thermal and oxidative stability of the final polyester product.
While the use of moderate temperatures of 100.degree.-160.degree. C. tend to suppress DGEE formation, rate of esterification of aromatic carboxylic acids with the ethylene oxide reactants is not commercially attractive in the absence of a catalyst. Amine catalysts such as ammonia, primary amines, secondary amines, tertiary amines, quaternary ammonium compounds (i.e. quaternary ammonium halides), N-heterocyclic amines; have been suggested as useful catalysts for example in French Pat. No. 1,428,204; British Pat. No. 623,669; U.S. Pat. No. 2,932,622 and U.S. Pat. No. 3,414,608. These patents also teach the use of inert reaction media such as water, acetone, methylethyl ketone, cyclohexanone, benzene, toluene, xylene, C.sub.6 to C.sub.10 alkane hydrocarbons, halogenated alkane hydrocarbons, ethers and preformed 2-hydroxyethyl esters including lower oligomers of the esters from dicarboxylic acids.
Chromium (III) catalysts (e.g. chromium octanoate) are broadly suggested for the reaction of aromatic carboxylic acids and oxirane compounds (i.e., ethylene and 1,2-propylene oxides) at temperatures of 25.degree.-250.degree. C. in Dutch Published Patent Application No. 67-01261, published July 27, 1967 and Belgian Pat. No. 715,201. Specific chromium (III) catalysts are organic compounds, for example Cr (III) 1-ethylpentane carboxylate, heptanecarboxylate, octanoate, oleate, cresylate, naphthenate, alkylbenzoate, alkoxybenzoate or chelate. Many of the above organic reaction media are also suggested as useful for this catalyzed reaction.
Our investigation of the catalytic effect of amine catalysts and chromium (III) catalysts with respect to aromatic acid esterification by ethylene and 1,2-propylene oxides indicated Cr (III) catalysts to produce a higher reaction rate (greater acid esterification in the same reaction time) at the same reaction temperatures but also produce DGEE at a higher rate. It has now been found that DGEE production enhanced by Cr (III) catalysts can be suppressed by the co-use of an amine without sacrifice of desired ester.